What advantages does Fly-Mode have over Walk-Mode?
There are some key differences that make fly mode better in some situations.


Motions are smoother turning and changing direction are not instant, this may be nicer for recording camera animations.
Speed is handled in a relative manner, so you dont have a fixed speed, it means you can naturally navigate large scenes or a small room in a large scene.
Some controls are different, MMB pans for example.
Walk mode isn't compatible with absolute pointing devices, eg - tablets.
Walk mode uses fixed horizon angle, fly mode allows navigation in all directions with optional z-up alignment.